The overall objective of this project is to understand the mechanisms by which aging pathological conditions or drug interactions alter the sensitivity of heart muscle to therapeutic and toxic effects of the digitalis glycosides. This is important because the digitalis glycosides are still widely used despite their narrow margin of safety, and Ca2 overload is a potential mechanism for cardiotoxicity of digitalis and also many agents and pathological conditions. Understanding the mechanisms by which cellular Ca2 overload causes myocardial dysfunctions and also regulation of sarcolemmal sodium pump under physiological and pathological conditions are essential for understanding of basic biology of the heart muscle. During the proposed project period, following studies will be performed: (1) The hypothesis that reduced Na pump capacity accounts for reduced tolerance of senescent heart to digitalis will be examined. Our previous data suggest that leak Na influx is increased and the number of active Na pumping sites is decreased in the myocardium of senescent Fischer 344 rats. Therefore, passive fluxes of Na, K and Ca2 across the sarcolemma, reserve capacity of the Na pump and possible causes of changes in the number of active Na pump units observed in senescent heart will be examined primarily in myocytes obtained from rat, guinea pig or dog heart. (2) The hypothesis that Ca2 overload of sarcoplasmic reticulum and subsequent uncontrolled Ca2 release are responsible for all phases of digitalis toxicity (contracture and reduction of developed tension, after contractions and oscillatory afterpotentials) will be critically evaluated in atrial muscle and cardiac Purkinje fibers using Ca2 entry blockers, Na/Ca2 exchange inhibitor and ryanodine. (3) Recently, the possible regulation of Na,K-ATPase by Ca2, H and inorganic phosphate has been suggested. Therefore, effects of changes in intracellular concentrations of these ions, ischemia and hypoxia and myocardial hypertrophy on the sarcolemmal Na pump and on glycoside binding to Na,K-ATPase will be examined using viable myocytes. Underlying hypothesis for this part of the project is that Ca2 regulates the Na pump under pathological conditions. Seemingly diverse aims of the proposed project converge into the testing of a unifying hypothesis that myocardial Na pump and its glycoside sensitivity are regulated by Na, K and Ca2 under physiological or pathological conditions, and to assess the biological impact of sodium pump inhibition on myocardial functions or dysfunctions.